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					                                                                                  DETECTORS
                                                                Photo Ionization Detector - PID

                                                 Overview
                                    The Photo Ionization Detector (PID) responds to all molecules whose ionization
                             potential is below 10.6eV, including aromatics and molecules with carbon double
                             bonds. The PID is nondestructive, so the sample can be routed through the PID
                             and on to other detectors. It is often used in series with the FID and / or DELCD.
         Benzene
                             PID detection limits for aromatics are in the ppb range; purge and trap concentration
                             of the sample can lower detection limits to the ppt range. Because of its selective
sensitivity, use of the PID is mandated in several EPA methods. The PID detector consists of a 10.6 electron
volt (eV) UV lamp mounted on a thermostatted, low-volume (100µL), flow-through cell. The temperature is
adjustable from ambient to 250oC. Three detector gain levels (LOW, MEDIUM and HIGH) are provided for
a wide range of sample concentrations. The PID lamp is held in place by a spring-loaded plate, so that the
lamp may be quickly removed for cleaning and replaced without any special tools. The PID can run on air
carrier for gasless operation, or for stream monitoring applications where the entire stream of sample is directed
through the detector (no column is used).




                                                                          110 PID
                                                                    stand-alone detector


                                          PID in series
                                          with a DELCD
                                          and an FID on
                                          an SRI 8610C
                                          chassis




                                          Spring-loaded          High voltage band in black plastic
      100µL detector cell                 retaining plate               hood over lamp anode ring
      (the area in the
      center of the
      TeflonTM seal ring)

           Heater block
                                                                                     BNC connector to amplifier
      Collector electrode
      support union                                                                    Amplifier gain switch
      Collector electrode
DETECTORS
Photo Ionization Detector - PID

                                          Theory of Operation
  10.6eV PID Lamp                                                                           Partial PID Assembly -
                                     The SRI PID design uses a 10.6eV lamp
(SRI Part # 8670-1242)                                                                         Exploded View
                                 with a high voltage power supply. Sample laden
                                 carrier gas flows from the analytical column into
                                 the PID sample inlet, where it is streamed
                                 through a 100µL flow-through cell. When
                                 sample molecules flow into the cell, they are
                                                                                                       PID lamp
                                 bombarded by the UV light beam. Molecules
                      Anode
                      ring       with an ionization potential lower than 10.6eV
                                 release an ion when struck by the ultraviolet
                                 photons. These ions are attracted to a collector              TeflonTM seal
                                 electrode, then sent to the amplifier to produce              (SRI part #
                                 an analog signal, which is acquired by the                    8670-1244)
                                 PeakSimple data system.
                                     Unlike other PID designs that heat the entire
                     Cathode     lamp, only the lamp window of the SRI PID is                    Collector
                     base        heated. This results in a longer lamp life for                  inlet
                                 SRI PID detectors.

                                  Simplified PID Operational Diagram
                                                                                               NOTE: The end of the
                                                                                               column must be visible in
                                                                TeflonTM     Spring-loaded     the detector cell when the
                                       100µL PID                  seal       retaining plate   PID lamp is removed from
                    Ferrule            detector cell                                           the retaining plate. It should
                                                                                               be approximately 1mm from
Sample-laden       Analytical                                                                  the lamp window when the
carrier gas         column                                                                     PID lamp is in place.
inlet (from the
column oven                                                                         UV
on 8610 &                                                                           light
310 GC’s, or
the heated
transfer line on
110 models)                                                                                    High voltage band inside
                                                        Ions are                               the black plastic hood
                                                        attracted                              (must make contact with
The PID cell effluent flows around the                  to the                                 the lamp anode for PID
column back to the PID sample gas                       collector                              operation; do not adjust
outlet, which is connected to the next                  electrode                              unless the main GC power
detector in series or vented to                                                                is turned OFF)
atmosphere inside the column oven                      Collector
                                                       electrode        Collector
                                                                        electrode           PID high
                                                       support
                        Heater block                                                         voltage
                                                       union
                                                                                                lead
                   Column oven wall
                                                       Collector electrode
                                                              signal cable
                                                                          DETECTORS
                                                        Photo Ionization Detector - PID


                                     Expected Performance

                                                                              PID Noise Run

                                                                        Column: 15m MXT-VOL
                                                                        Carrier: Helium @ 10mL/min
                                                                        PID gain: LOW
                                                                        PID temp: 150oC
                                                                        PID current: 70




            PID noise averages less than 50µV from peak to peak




                           PID BTEX Analysis (in series with FID and DELCD)

Sample: 1µL 100ppm
BTEX plus
Column: 15m MXT-VOL
Carrier: Helium @ 10
mLs/min
PID gain: LOW
PID temp: 150oC
PID current: 70

Temperature program:
Initial Hold Ramp Final
40oC 2.00 15.00 240oC
240oC 10.00 0.00 240oC

Results:
Component     Retention Area
Benzene       2.416     313.0540
TCE           3.066     231.5120
Toluene       4.600     309.2120
PCE           5.433     216.6230
Ethyl Benzene 6.700     286.0900
Ortho Xylene 7.383      298.9190
Bromoform     7.650      55.9460
           Total       1711.3560
DETECTORS
Photo Ionization Detector - PID


                                General Operating Procedure

    The capillary column enters the PID cell from inside the column oven through the bulkhead fitting in the
insulated oven wall. The column may be installed with the lamp in place. Insert the capillary column into the
PID detector inlet until the column stops at the lamp window inside the PID cell, then pull it back about 1mm
from the lamp window. Tighten the 1/8” nut with the graphite ferrule at the PID inlet to secure the column in
place. The collector electrode is positioned at the factory and should not touch the column under normal
circumstances.



1. Always ensure that the black plastic hood is in place on the lamp prior to operating the PID detector. The
hood contains the high voltage band which is maintained at a high potential; never attempt to adjust the PID
high voltage band unless the main GC power is turned off.
                                                         The violet light is visible here when the lamp is on
2. Turn ON the GC. Turn ON the PID lamp current
with the flip switch on the GC’s front control panel.

3. Set the PID current to 70 (= 0.70ma) with the
trimpot setpoint on the top edge of the GC’s front
control panel. Use the flat blade screwdriver
provided with your GC to adjust the trimpot. The
lamp should emit a violet-colored light visible down
the center of the tube.
                                                                    PID - Close-up topview
4. Confirm that the lamp is operating at or near
0.70ma by pressing the PID detector ACTUAL
display button on the front control panel. The sensitivity of the lamp increases proportionally to the current
applied, but operation at higher currents reduces lamp life. The PID operating current range is 70-125. A
setting of 70 should provide the user with sufficient sensitivity and lamp durability. Most PID applications can
be performed using LOW gain.

5. Set the PID temperature to 150oC.

6. Once the detector has reached temperature and the signal appears stable, sample may be introduced.




NOTE: Lamps are a consumable part of the PID detector. It is recommended to have a spare lamp available
if critical analyses are being performed at remote field sites. Spare and replacement 10.6eV PID lamps are
available under SRI part number 8670-1242. Teflon seals are available under SRI part number 8670-1244.
                                                                                 DETECTORS
                                                               Photo Ionization Detector - PID

                            Troubleshooting and Maintenance
                                         Cleaning the PID Lamp
                                                                                          The PID lamp window
    Over time, during normal operation, a film of contaminants will condense on the
PID lamp window. Typically, this film is a result of stationary phase column bleed. To
minimize contaminant condensation and thus lamp window cleaning, avoid heating the
column any higher than absolutely necessary. Contaminant condensation can block
the photons, reducing lamp emissions and sensitivity. Therefore, the PID lamp window
must be cleaned when an appreciable change in sensitivity has been observed by the
operator. Because the response change resulting from cleaning the lamp window
usually requires detector recalibration, frequent cleaning is not recommended.

1. Turn the PID current OFF with the switch on the GC’s front control panel. Turn the GC OFF and let the
PID detector assembly cool enough to touch it without getting burned.

2. Disconnect the high-voltage band from the lamp anode by removing
the black plastic hood.

3. Grasp the spring-loaded retainer plate with the fingers of one hand
and push or pull it toward the PID lamp; it doesn’t take much force to
move the plate enough for lamp removal. Slide the PID lamp up and out
of the PID detector assembly.

4. Clean the lamp window using a mild abrasive cleanser like Bon Ami
or Comet. Wet your finger, and make a paste with a small amount of
cleanser. Scrub the lamp window clean in a circular motion with your finger.

5. Rinse the lamp window clean with water. Dry the lamp with a paper towel.

6. Inspect the TeflonTM seal for cuts or nicks. A damaged seal will not affect the       The collector electrode
                                                                                         protrudes into the cell
PID response, but it may provide a leak site that will reduce the amount of
sample delivered to any subsequent detector.

7. With the lamp removed, the collector electrode is visible where it protrudes
into the cell. Check the collector electrode for any visible residues, films,
discolorations, etc. If present, they may impede the flow of ions from the sample
molecules to the collector electrode. To clean the collector electrode, gently use           TeflonTM seal
a small file to remove any residues from its tip. Blow the residue
off the collector electrode and surrounding areas.                       Make sure the lamp window is centered
                                                                        over the Teflon seal and snug against it
8. Open the spring-loaded retainer plate and replace the PID
lamp snug against the seal. The lamp window has a slightly larger
diameter than the seal; try to center it against the seal. Replace
the high voltage band / black plastic lamp hood.

9. Recalibrate the PID detector before returning it to service.

				
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